The attenuation of noise in a closed structure or body created by a source disposed either externally or in the interior of the enclosure, has to this point generally been accomplished by so called passive means of attenuation. As used herein, the term closed structure refers generally to an enclosure having an interior bounded by essentially continuous walls, such as, for example, a room with its doors and windows closed or an airplane fuselage with its exit doors closed. Passive attenuation of sound in such applications has been accomplished by disposing one or more layers of material, such as barrier materials, absorbing materials and damping materials, between the source of the sound and the area where a reduced noise level is desired. For example, assume sound is produced within a closed room or other structure by a source exterior to the enclosure. A typical configuration of passive attenuating materials to achieve a reduced noise level in the enclosure may include an outermost layer of barrier material having a high density disposed adjacent to or at the boundary layer of the enclosure. The high density barrier material reflects at least some of the sound waves propagating from the exterior source of noise outwardly, away from the enclosure. Extending inwardly from the boundary layer in many passive attenuating configurations is a layer of acoustically absorbent material, such as fiberglass, which acts to extract energy from the source sound waves which reflect from the outer barrier material toward the interior of the enclosure. In some applications, the passive means of acoustic attenuation may also include damping materials disposed adjacent the acoustically absorbent material and toward the exterior of the enclosure. Damping materials, such as damping tape and the like, extract further energy from the remaining source sound waves before they enter the interior of the enclosed structure.
Passive means of sound attenuation such as described above provide adequate reductions in noise levels for a variety of applications. However, in other applications, passive attenuating materials are of limited utility. Considering the application of passenger aircraft fuselages, which will be discussed herein to illustrate the advantages of this invention, passive means of noise attenuation create as well as solve problems. As mentioned above, acoustically reflective barrier materials must be relatively dense to be effective in reflecting incident sound waves. The higher the density of a material the more it weighs. It is apparent that the addition of weight to the fuselage of a passenger aircraft to enhance noise attenuation has the adverse affect of reducing fuel economy, payload and flight range. In addition, most acoustically absorbent or damping materials are relatively easily damaged and make poor surfaces for use in the interior of aircraft.
There have been limited efforts in the prior art to achieve reduced sound levels in the interior of enclosed structures in those applications where passive means of attenuation present functional problems. One approach to the attenuation of noise within the interior of an aircraft fuselage, for example, is found in Bschorr U.S. Pat. No. 3,685,610. In this patent, transmitters located externally of the fuselage adjacent the aircraft propeller are operable to produce sound waves having the same frequency and amplitude but of opposed phase to that of the sound produced by the propellers and engines. This is the same general approach taught in Connover U.S. Pat. No. 2,776,020 which involves the attenuation of transformer noise. These designs are directed to the attenuation of sound waves from an exterior source at or near the source before such sound waves can propagate to an area such as a closed structure where a reduced noise level is desired.
A second approach to the attenuation of sound in an aircraft fuselage is found in Vang U.S. Pat. No. 2,361,071 which is directed to a means of reducing aircraft vibration produced by the engines and propellers at a point on or adjacent to the fuselage. In this design, vibration attenuation means are randomly disposed within the interior of the aircraft fuselage. The attenuation means include a displacement type vibration pick-up for sensing the vibration of the fuselage during flight, which pick-ups are adapted to operate electric vibrators mounted to the interior of the fuselage skin for the production of vibration opposed to that acting on the exterior surface of the fuselage. There is no disclosure provided as to the preferred locations of such vibration damping means along the fuselage and it appears that significant difficulty would be encountered in achieving a balance in the vibrations where the units are located throughout the fuselage. In addition, the number of units apparently required would appear to make this approach costly and inefficient.
It is therefore an object of this invention to provide an active means of attenuation of the noise within a closed structure.
It is another object herein to provide an active system for the attenuation of noise within an enclosure such as a closed structure or body produced by a source or sources of noise disposed either exteriorly of or within the enclosure.
It is a further object of this invention to provide an active system for the attenuation of noise within a closed structure produced by a source of noise exterior to the structure, which involves the equalization of the pressure exerted by the source sound waves on the exterior surface of the enclosure.
It is another object of this invention to provide an active attenuator system for the reduction of noise levels within a closed structure in which all elements of the attenuator system are disposed at high acoustic pressure anti-nodes within the structure.